I'd strongly recommend multiple students per class for young kids, or they will often lose interest.
Yep, I was thinking of tutoring two at the same time, and then playing learning games at 30 minute or 1 hour intervals so that the game, and the competition, holds their interest.
Depends on the kid, depends on the tutoring session length. I've been teaching primary kids for several years now and the more of them there are, the more they'll ignore you and get distracted because they still lack the focus that we have.
If you're going to be preparing booklets and putting effort in, tbh, I wouldn't be doing it for only $15
How many kids do you teach at a time session? Do you think that they'd get distracted if there were 2-3?
Also, I'm not sure how much I should be charging if it's in a group...if I said $15 per hour for one-on-one...should it be $15 each for a group sesh? I'll feel as if I'm overcharging, especially since they're family friends.
But then again, you're right, making the booklets do take ages, so if I take those hours into account, it wouldn't be ridiculous to charge more right? Luckily it's still the holidays so I might be able to make a couple more before school starts. Do you recommend that I buy some Excel Basic Skills books (or some other of the sort) to use when teaching them?
Anyway, you really need to work out what she expects to get out of this.
I would also recommend that before your sessions you revise long division (2, 3, 4 digit), fraction multiplication/division etc. because whilst you might do them in your head, you have to explain them stepwise to the kid - can you do that right now? I couldn't when I first started 
Great advice, thanks so much Russ! You're a legend.

I'll definitely prepare for all my sessions.
(Although he's not going for scholarship tests this year, I think his mother wants him to the year after. She said she's willing to pay for bi-weekly tuition in the future, and although I've got experience of the whole process, and I know that I will try my best to guide and teach him, I can't promise that I'll be able to help him get in, because doing well in those tests depends largely on natural ability...which I'm uncertain he has.

How should I tell her that I can't make any guarantees? Do you have any experience of teaching kids for scholarship exams?